This invention is concerned with fiber-reinforced composites. It is particularly concerned with composites embodying graphite or silicon carbide type fibers as reinforcement for a glass or ceramic type matrix.
Graphite/glass composites are well known. Several recent patents (U.S. Pat. No. 4,265,968, U.S. Pat. No. 4,263,367, U.S. Pat. No. 4,256,378, U.S. Pat. No. 3,681,187 and U.S. Pat. No. 3,607,608) are concerned with such composites. These employ a borosilicate glass, available from Corning Glass Works under Code No. 7740, as an exemplary matrix glass.
The reinforcement provided by fibers is usually evaluated in terms of material strength. A measure commonly used is ultimate tensile strength. This measure is contributed to by both the inherent strength of the fibers and the bond that is engendered between the fibers and the matrix being strengthened.
The quality of a fiber-reinforced composite may also be evaluated in terms of fracture toughness and fatigue resistance. These properties depend on inherent fiber strength and bond strength as well, but also reflect micro-cracking tendencies in the matrix. More accurately, they reflect the resistance to such tendencies.
As further illustrated later, toughness may be evaluated from the shape of a load deflection curve generated when a test sample is progressively loaded in a bending apparatus to the point of fracture. Maximum stress may then be calculated. The ascending curve for a strong and/or brittle composite tends to drop precipitously when the maximum load is reached. In contrast, the curve for a tough material tends to drop rather gradually. Physically, this is manifested as a non-spreading, or slowly developing, crack.